Thermal management in electronic devices with yielding substrates

ABSTRACT

In accordance with certain embodiments, heat-dissipating elements are integrated with semiconductor dies and substrates in order to facilitate heat dissipation therefrom during operation.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/788,915, filed Mar. 15, 2013, the entiredisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to thermal management ofelectronic devices, and more specifically to thermal management inarray-based LED lighting systems.

BACKGROUND

Discrete light sources such as light-emitting diodes (LEDs) are anattractive alternative to incandescent light bulbs in illuminationdevices due to their higher efficiency, smaller form factor, longerlifetime, and enhanced mechanical robustness. However, the high cost ofLEDs and associated heat-sinking and thermal-management systems havelimited the widespread utilization of LEDs, particularly in generallighting applications.

The high cost of LED-based lighting systems has several contributors.LEDs are typically encased in a package and multiple packaged LEDs areused in each lighting system to achieve the required light intensity. Inorder to reduce costs, LED manufacturers have developed high-power LEDs,which can emit relatively higher light intensities by operating atrelatively higher currents. While reducing the package count, these LEDsrequire relatively higher-cost packages to accommodate the highercurrent levels and to manage the significantly higher heat levels thatresult. The higher heat loads and currents, in turn, require moreexpensive thermal-management and heat-sinking systems that also add tothe cost as well as to size of the system. Higher operating temperaturesalso lead to shorter lifetimes, degradation in luminous output, andreduced reliability.

Conventional LED thermal-management and heat-sinking systems have beendevised to improve the heat transfer from the light-emitting junction inthe LED to external heat-dissipating elements. Heat-dissipating elementstypically include thermal slugs in the package, ceramic or metalsubmounts, large metal or ceramic heat sinks, and the like. Anotherexample of a conventional heat-dissipating element is a high thermalconductivity wiring board, such as a metal core printed circuit board.Thermally conductive underfills may also be used to aid in transfer ofheat from the package to the wiring board.

In conventional systems, the electrical traces may be utilized asheat-dissipating elements; in such systems, the LED typically has a pairof electrical contacts in electrical and thermal engagement with thetraces, which are typically composed of a metallic material. Heatgenerated by the LED flows through its electrical contacts into thetraces. Heat may also flow out the body of the LED, and be dissipatedthrough an underfill that has a relatively high thermal conductivity.Typical underfills include thermally conductive adhesives or pastes.Typically, the traces and the underfill are thermally coupled to a highthermal conductivity substrate, such as AlN, a metal core printedcircuit board, or other high thermal conductivity material. Because thesubstrate has high thermal conductivity there is a relatively largearea—approximately the entire chip area—through which heat may flow fromthe chip to the substrate. The high thermal conductivity substratedissipates heat transferred to it and this substrate is therefore animportant element in thermal management of high-power LED-based lightingsystems.

An alternative approach to conventional high-power LED-based lightingsystems is the use of an array of relatively small LEDs mounted on a lowcost plastic base substrate and driven at relatively low current.Examples of such systems are disclosed in U.S. Pat. No. 8,384,121, filedJun. 29, 2011 (the '121 patent), U.S. Patent Application Publication No.2014/0062318, filed Mar. 13, 2013, and U.S. Patent ApplicationPublication No. 2014/0062316, filed Aug. 19, 2013, the entire disclosureof each of which is incorporated herein by reference. Such systemstypically feature conductive traces, formed over a plastic substrate,and which are used to interconnect and provide power to an LED throughits electrical contacts. When the LEDs on the plastic substrate areoperated at low drive currents, no additional thermal management may berequired, because the relatively small amount of heat generated by eachLED is distributed over the entire array area. Such heat distributionmay be through the conductive traces, which typically have a relativelylarger thermal conductivity than the plastic substrate. Thus, whilethese systems typically have significantly lower cost because there isno need for additional heat sinking or thermal management, they may belimited to relatively low drive currents.

In view of the foregoing, a need exists for systems and techniquesenabling the design and manufacture of LED-based lighting systems, aswell as corresponding thermal-management techniques, capable ofsupporting relatively high LED drive currents at low cost.

SUMMARY

In accordance with various embodiments, the invention provides anelectrically driven lighting system design that features an electricallyinsulating (e.g., plastic) substrate having opposed first and secondsurfaces with circuit traces that are electrically conductive. Thesubstrate contains one or more heat-dissipating devices disposed uponand in parallel relationship with the substrate. Multiple light-emittingelements (LEEs), for example, LED semiconductor dies, are disposed onthe substrate, and each LEE has a pair of (or more) electrical contacts.Each LEE may also include heat-dissipating devices, as described in moredetail below. In general, each LEE is in thermal communication with oneor more heat-dissipating devices for conveying heat from the LEE to theheat-dissipating device(s) and thus away from the LEE. Embodiments ofthe invention also include techniques for fabricating the assemblyincluding the substrate, LEEs, and heat-dissipating devices. Suchtechniques may include disposing each LEE onto the substrate andbringing the LEE and one or more heat-dissipating devices into thermalcontact.

In some embodiments, the LEE has a p-contact that is asymmetricallylarger than the n-contact, in order to enable the greater transfer ofheat from the p-region of the LED where more surface area is availablefor heat transfer. The asymmetrically larger p-contact generallytransfers heat more effectively than the n-contact, because thep-contact directly underlies the region below the junction where lightand heat are produced in the LEE. The heat transfer from the p-regionfollows the shortest path through the p-contact and intoheat-dissipating elements such as the conductive traces and thesubstrate. The p-contact therefore has, in various embodiments of theinvention, superior heat transfer ability relative to the n-contact. (Inembodiments in which the n-region of the LEE underlies thelight-producing junction, the n-contact may be asymmetrically largerthan the p-contact.)

In various embodiments, relatively large electrical contacts areutilized on the LEE to increase the reliability of the LEE arrayassembly, because in general a larger contact surface area yields agreater probability of sufficient electrical and thermal bonding of theLEE onto the conductive traces during die attachment. LEEs havingasymmetrically larger p-contacts may be placed in an offset position onthe conductive traces in order to increase the electrical contact areaand thermal transfer region between the asymmetrically larger p-contactand its corresponding conductive trace. Specifically, the larger contactof the LEE may overlap its corresponding conductive trace to a greaterextent than the smaller contact of the LEE.

In some embodiments, the gap between each pair of conductive traces onthe substrate is minimized. A smaller gap between the conductive tracesmay increase the contact area and thus reduce the thermal resistancebetween the LEE contacts and the conductive traces. For example, in someembodiments, the gap between the conductive traces may be less than 300μm, or less than 200 μm, or less than 100 μm, or even less than 50 μm or25 μm. A smaller gap between the conductive traces may increase thereliability of the LEE array assembly by providing more misalignmenttolerance of electrically-coupled and thermally-coupled LEE contactsonto the conductive trace. A narrower gap may also reduce themanufacturing tolerance in the bonding process of LEEs onto thesubstrate. As a result, fewer bonding errors occur during the LEE arrayassembly process.

For LEEs having asymmetrically sized contacts, the smaller (e.g., n-)contact may feature a via perpendicularly disposed therethrough in orderto provide greater surface area in the corresponding semiconductor(e.g., n-) region and enable greater transfer of heat from that regionof the LEE, through the contact, and into the conductive trace andsubstrate. Such vias may also be utilized for larger contacts and forone or both contacts in embodiments in which the contacts aresubstantially the same size.

In various embodiments, one or more LEE thermal pads that are not inelectrical communication with the either the p-contact or the n-contactare disposed between and isolated from the p-contact and the n-contact(e.g., formed atop an isolation layer). An isolation layer under an LEEthermal pad may be very thin, as it is under the LEE thermal pad andthus needs not be as robust to survive the die-attach assembly process.In some embodiments, the isolation layer thickness is less than 0.25 μm,less than 0.1 μm, less than 0.05 μm, less than 0.02 μm, or less thaneven 100 Å or 50 Å.

In some embodiments, an LEE thermal pad includes or consists essentiallyof a relatively high thermal conductivity material such as polysilicon,poly SiC, poly AlN, SiC, AlN, and amorphous versions of these and othermaterials. In some embodiments, the LED thermal pad includes or consistsessentially of one or more relatively high thermal conductivity metalssuch as gold, copper, solder, aluminum, silver, carbon, silver ink,carbon ink, or the like. In some embodiments, the LEE thermal pad has awidth that is less than the spacing between the conductive traces towhich the LEE is bonded, thus preventing the LEE thermal pad fromelectrically shorting the conductive traces. In other embodiments, theLEE thermal pad has a width that is greater than the spacing between theconductive traces to which the LEE is bonded, and it does contact onebut not both of the conductive traces. However, in such embodiments theLEE is offset relative to the gap centerline, thus preventing the LEEthermal pad from electrically shorting the conductive traces.

In accordance with various embodiments of the present invention, asubstrate thermal trace material is placed or formed on the substrate inthe gap between the conductive traces (to which an LEE is bonded) inorder to enhance thermal conduction from the LEE thermal pad into thesubstrate. In some embodiments, the substrate thermal trace material mayinclude or consist essentially of the same material as that of thesubstrate, or the substrate thermal trace material may be a differentmaterial. In various embodiments, the substrate thermal trace materialis a material that has a relatively high thermal conductivity such aspolysilicon, poly SiC, poly AlN, SiC, AlN, and amorphous versions ofthese and other materials. Embodiments of the invention may utilize oneor more LEE thermal pads with or without the use of the substratethermal trace material.

The substrate thermal trace material may be the same material as theconductive traces on the substrate. However, in other embodiments, thesubstrate thermal trace material may be substantially electricallyinsulating. In yet additional embodiments, a relatively high thermalconductivity material that is substantially electrically isolating, forexample an adhesive or paste, is placed or formed in the area around thesubstrate thermal trace material.

In various embodiments of the invention in which large amounts of heatare to be channeled away from LEEs, the heat-dissipation devicesdetailed herein may themselves be thermally (and even mechanically)coupled to larger bulk heat sinks (e.g., high-surface-area and/or finned“blocks” of a thermally conductive material) and/or to activeheat-removal devices such as fans.

As utilized herein, the term “light-emitting element” (LEE) refers toany device that emits electromagnetic radiation within a wavelengthregime of interest, for example, visible, infrared or ultravioletregime, when activated, by applying a potential difference across thedevice or passing a current through the device. Examples of LEEs includesolid-state, organic, polymer, phosphor-coated or high-flux LEDs,microLEDs (described below), laser diodes or other similar devices aswould be readily understood. The emitted radiation of a LEE may bevisible, such as red, blue or green, or invisible, such as infrared orultraviolet. A LEE may produce radiation of a spread of wavelengths. ALEE may feature a phosphorescent or fluorescent material for convertinga portion of its emissions from one set of wavelengths to another. A LEEmay include multiple LEEs, each emitting essentially the same ordifferent wavelengths. In some embodiments, a LEE is an LED that mayfeature a reflector over all or a portion of its surface upon whichelectrical contacts are positioned. The reflector may also be formedover all or a portion of the contacts themselves. In some embodiments,the contacts are themselves reflective.

An LEE may be of any size. In some embodiments, an LEE has one lateraldimension less than 500 μm, while in other embodiments an LEE has onelateral dimension greater than 500 μm. Exemplary sizes of a relativelysmall LEE may include about 175 μm by about 250 μm, about 250 μm byabout 400 μm, about 250 μm by about 300 μm, or about 225 μm by about 175μm. Exemplary sizes of a relatively large LEE may include about 1000 μmby about 1000 μm, about 500 μm by about 500 μm, about 250 μm by about600 μm, or about 1500 μm by about 1500 μm. In some embodiments, an LEEincludes or consists essentially of a small LED die, also referred to asa “microLED.” A microLED generally has one lateral dimension less thanabout 300 μm. In some embodiments, the LEE has one lateral dimensionless than about 200 μm or even less than about 100 μm. For example, amicroLED may have a size of about 225 μm by about 175 μm or about 150 μmby about 100 μm or about 150 μm by about 50 μm. In some embodiments, thesurface area of the top surface of a microLED is less than 50,000 μm² orless than 10,000 μm². The size of the LEE is not a limitation of thepresent invention, and in other embodiments the LEE may be relativelylarger, e.g., the LEE may have one lateral dimension on the order of atleast about 1000 μm or at least about 3000 μm. In some embodiments theLEE may emit white light or substantially white light.

In some embodiments, various elements such as substrates or lightsheetsare “flexible” in the sense of being pliant in response to a force andresilient, i.e., tending to elastically resume an original configurationupon removal of the force. Such elements may have a radius of curvatureof about 1 m or less, or about 0.5 m or less, or even about 0.1 m orless. In some embodiments, flexible elements have a Young's Modulus lessthan about 100 N/m², less than about 50 N/m², or even less than about 10N/m². In some embodiments, flexible elements have a Shore A hardnessvalue less than about 100; a Shore D hardness less than about 100;and/or a Rockwell hardness less than about 150.

In an aspect, embodiments of the invention feature an electronic devicethat includes or consists essentially of a substrate, a light-emittingdiode (LED), and a thermal pad. The substrate has first and secondconductive traces on a first surface thereof, and the first and secondconductive traces are separated on the substrate by a gap therebetween.The LED has first and second distinct electrical contacts on a first LEDsurface thereof, and the first and second contacts are attached andelectrically coupled to, respectively, the first and second conductivetraces. At least a portion of the thermal pad is disposed within the gapbetween the first and second conductive traces. At least a portion ofthe thermal pad is disposed between the first surface of the substrateand the LED. The thermal pad is electrically isolated from the firstand/or second electrical contacts. The thermal pad thermally couples theLED to the substrate without electrically coupling the LED to thesubstrate, and the thermal pad may have a thermal conductivity greaterthan 75 W/(m·K), or even greater than 150 W/(m·K).

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. The thermal pad may bepartially disposed over only one of the first or second conductivetraces but not the other. A surface area of the thermal pad may begreater than a surface area of the first electrical contact and/or asurface area of the second electrical contact. The thermal pad may bebonded to the first surface of the substrate and to the first LEDsurface. The thermal pad may be bonded to the first surface of thesubstrate but not bonded to the first LED surface. The thermal pad maybe bonded to the first surface of the substrate via a thermallyconductive adhesive, thermally conductive epoxy, adhesive, anisotropicconductive adhesive, thermal grease, and/or a thermal gasket. Thethermal pad may be spaced away from and not in direct contact with theLED. The thermal pad may be bonded to the first LED surface but notbonded to the substrate. The thermal pad may be spaced away from and notin direct contact with the substrate. The thermal pad may be in directcontact with the first surface of the substrate.

The thermal pad may be bonded to the first surface of the substrate, anda second thermal pad may be bonded to the first LED surface and disposedbetween the thermal pad and the first LED surface. The second thermalpad may be attached and thermally coupled to the thermal pad. The secondthermal pad may be attached to the thermal pad via a thermallyconductive adhesive, thermally conductive epoxy, adhesive, anisotropicconductive adhesive, thermal grease, and/or a thermal gasket. A surfacearea of the thermal pad may be at least two times, at least five times,or at least ten times a surface area of the second thermal pad. Thethermal pad may include or consist essentially of aluminum, copper,gold, silver, carbon, silver ink, copper ink, and/or carbon ink, and thesecond thermal pad may include or consist essentially of gold, siliconcarbide, and/or aluminum nitride. The thermal pad may be electricallyisolated from the first and/or second conductive traces. The thermal padmay include or consist essentially of a material having a thermalconductivity in the range of about 150 W/(m·K) to about 600 W/(m·K). Thethermal pad may include or consist essentially of aluminum, copper,gold, silver, carbon, silicon carbide, aluminum nitride, silver ink,copper ink, and/or carbon ink.

A backside thermal pad may be disposed on a second surface of thesubstrate opposite the first surface. At least one thermal via mayextend from the first surface of the substrate to the second surface ofthe substrate and thermally couple the thermal pad to the backsidethermal pad. The backside thermal pad may have a thermal conductivitygreater than 75 W/(m·K), or even greater than 150 W/(m·K). The at leastone thermal via may be bonded to the thermal pad and to the backsidethermal pad. The at least one thermal via may include or consistessentially of aluminum, copper, gold, silver, carbon, carbon ink,silver ink, copper ink, silicon, silicon carbide, and/or aluminumnitride. The backside thermal pad may include or consist essentially ofaluminum, copper, gold, silver, carbon, silicon, carbon ink, silver ink,copper ink, silicon carbide, and/or aluminum nitride. The backsidethermal pad may include or consist essentially of a material having athermal conductivity in the range of about 150 W/(m·K) to about 600W/(m·K). The at least one thermal via may include or consist essentiallyof a material having a thermal conductivity in the range of about 10W/(m·K) to about 600 W/(m·K). Each thermal via may have a thermalresistance in the range of about 0.05° K/W to about 10° K/W. The atleast one thermal via may include or consist essentially of (i) acrimp-type via between the first and second surfaces of the substrate,(ii) a staple extending through the substrate in at least two discretelocations, (iii) a rivet extending through the substrate, (iv) a clampextending around the substrate but not through the substrate, and/or (v)a wire. The at least one thermal via may include or consist essentiallyof a through-hole extending through the substrate and substantiallyfilled with a thermally conductive material having a thermalconductivity greater than 10 W/(m·K). The thermally conductive materialmay include or consist essentially of aluminum, copper, gold, silver,carbon, carbon ink, silver ink, copper ink, conductive adhesive, and/orconductive epoxy.

The substrate may be flexible. The substrate may include or consistessentially of polyethylene naphthalate, polyethylene terephthalate,epoxy, polycarbonate, polyethersulfone, polyester, polyimide,polyethylene, and/or silicone. The first and second contacts may bebonded to the first and second conductive traces by an adhesive. Theadhesive may include or consist essentially of a pressure-activatedadhesive, an anisotropic conductive adhesive, a conductive adhesive, anon-conductive adhesive, a heat-activated adhesive, and/or aUV-activated adhesive. The first and second contacts may be bonded tothe first and second conductive traces by solder. The thermal pad may bebonded to the first and second conductive traces by an adhesive. Theadhesive may include or consist essentially of a pressure-activatedadhesive, an anisotropic conductive adhesive, a conductive adhesive, anon-conductive adhesive, a heat-activated adhesive, and/or aUV-activated adhesive. The first and second contacts may be bonded tothe thermal pad by solder. The first and second conductive traces mayinclude or consist essentially of aluminum, copper, gold, silver,carbon, silver ink, and/or chromium.

The LED may include a wavelength-conversion material (e.g., disposedaround at least a portion of the LED semiconductor layer structureand/or within or on at least a portion of a package surrounding thesemiconductor layer structure) for absorbing a portion of the lightemitted by the LED and emitting light of a different wavelength. Thelight emitted by the LED and the light emitted by thewavelength-conversion material may combine to form substantially whitelight. The LED may include or consist essentially of one or more galliumnitride-based semiconductors (e.g., GaN, InGaN, InAlGaN, AlGaN, etc.).The LED may be part of one of a plurality of light-emitting stringsdisposed on the first surface of the substrate and interconnected in anelectric circuit. Each light-emitting string may include or consistessentially of a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string. The electronic device mayinclude a power source for energizing the plurality of light-emittingstrings and/or circuitry for controlling the current through theplurality of light-emitting strings. The LED may include or consistessentially of a bare-die LED or a packaged LED.

In another aspect, embodiments of the invention feature an electronicdevice that includes or consists essentially of a substrate, first andsecond conductive traces, a backside thermal pad, a thermal via, and alight-emitting diode (LED). The substrate has a first surface and asecond surface opposite the first surface. The first and secondconductive traces are disposed on the first surface of the substrate andseparated on the substrate by a gap therebetween. The backside thermalpad is on the second surface of the substrate. The backside thermal padmay have a thermal conductivity greater than 75 W/(m·K), or even greaterthan 150 W/(m·K). The thermal via extends from the first surface of thesubstrate to the second surface of the substrate and thermally couplesthe first conductive trace to the backside thermal pad. The thermal viamay have a thermal conductivity greater than 10 W/(m·K), or even greaterthan 75 W/(m·K), or even greater than 150 W/(m·K). The LED has first andsecond distinct electrical contacts on a first LED surface thereof. Thefirst and second contacts are attached and electrically coupled to,respectively, the first and second conductive traces.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. The thermal via may bebonded to the first conductive trace and to the backside thermal pad. Asecond backside thermal pad, distinct from the backside thermal pad, maybe disposed on the second surface of the substrate. A second thermal viamay extend from the first surface of the substrate to the second surfaceof the substrate and thermally couple the second conductive trace to thesecond backside thermal pad. The thermal via may be bonded to the firstconductive trace and to the backside thermal pad. The second thermal viamay be bonded to the second conductive trace and to the second backsidethermal pad. The thermal via may electrically couple the firstconductive trace to the backside thermal pad. A surface area of thethermal via substantially parallel to the first and/or second surfacesof the substrate may be in the range of about 0.1 mm² to about 10 mm².The thermal via may include or consist essentially of aluminum, copper,gold, silver, carbon, carbon ink, silver ink, copper ink, silicon,silicon carbide, and/or aluminum nitride. The backside thermal pad mayinclude or consist essentially of aluminum, copper, gold, silver,carbon, carbon ink, silver ink, copper ink, and/or silicon. The backsidethermal pad may include or consist essentially of a material having athermal conductivity in the range of about 150 W/(m·K) to about 600W/(m·K). The thermal via may include or consist essentially of amaterial having a thermal conductivity in the range of about 10 W/(m·K)to about 600 W/(m·K). The thermal via may have a thermal resistance inthe range of about 0.05° K/W to about 10° K/W. The thermal via mayinclude or consist essentially of (i) a crimp-type via between the firstand second surfaces of the substrate, (ii) a staple extending throughthe substrate in at least two discrete locations, (iii) a rivetextending through the substrate, (iv) a clamp extending around thesubstrate but not through the substrate, and/or (v) a wire. The thermalvia may include or consist essentially of a through-hole extendingthrough the substrate and substantially filled with a thermallyconductive material having a thermal conductivity greater than 10W/(m·K). The thermally conductive material may include or consistessentially of aluminum, copper, gold, silver, carbon, conductiveadhesive, and/or conductive epoxy.

The substrate may be flexible. The substrate may include or consistessentially of polyethylene naphthalate, polyethylene terephthalate,epoxy, polycarbonate, polyethersulfone, polyester, polyimide,polyethylene, and/or silicone. The first and second contacts may bebonded to the first and second conductive traces by an adhesive. Theadhesive may include or consist essentially of a pressure-activatedadhesive, an anisotropic conductive adhesive, a conductive adhesive, anon-conductive adhesive, a heat-activated adhesive, and/or aUV-activated adhesive. The first and second contacts may be bonded tothe first and second conductive traces by solder. The first and secondconductive traces may include or consist essentially of aluminum,copper, gold, silver, carbon, carbon ink, copper ink, silver ink, and/orchromium. The LED may include a wavelength-conversion material (e.g.,disposed around at least a portion of the LED semiconductor layerstructure and/or within or on at least a portion of a packagesurrounding the semiconductor layer structure) for absorbing a portionof the light emitted by the LED and emitting light of a differentwavelength. The light emitted by the LED and the light emitted by thewavelength-conversion material may combine to form substantially whitelight. The LED may include or consist essentially of one or more galliumnitride-based semiconductors (e.g., GaN, InGaN, InAlGaN, AlGaN, etc.).The LED may be part of one of a plurality of light-emitting stringsdisposed on the first surface of the substrate and interconnected in anelectric circuit. Each light-emitting string may include or consistessentially of a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string. The electronic device mayinclude a power source for energizing the plurality of light-emittingstrings and/or circuitry for controlling the current through theplurality of light-emitting strings. One or more additional LEDs may bedisposed on the first surface of the substrate. The electronic devicemay include a power source for energizing the LED and the one or moreadditional LEDs, and/or circuitry for controlling current through theLED and the one or more additional LEDs. At least a portion of the powerenergizing the LED or the one or more additional LEDs may be suppliedthereto through the backside thermal pad and thermal via.

The electronic device may include a thermal pad. At least a portion ofthe thermal pad may be disposed within the gap between the first andsecond conductive traces. At least a portion of the thermal pad may bedisposed between the first surface of the substrate and the LED. Thethermal pad may be electrically isolated from the first and/or secondelectrical contacts. The thermal pad may thermally couple the LED to thesubstrate without electrically coupling the LED to the substrate. Thethermal pad may have a thermal conductivity greater than 75 W/(m·K), oreven greater than 150 W/(m·K). The thermal pad may be partially disposedover only one of the first or second conductive traces but not theother. A surface area of the thermal pad may be greater than a surfacearea of the first electrical contact and/or a surface area of the secondelectrical contact. The thermal pad may be bonded to the first surfaceof the substrate and to the first LED surface. The thermal pad may bebonded to the first surface of the substrate but not bonded to the firstLED surface. The thermal pad may be bonded to the first surface of thesubstrate via a thermally conductive adhesive, thermally conductiveepoxy, adhesive, anisotropic conductive adhesive, thermal grease, and/ora thermal gasket. The thermal pad may be spaced away from and not indirect contact with the LED. The thermal pad may be bonded to the firstLED surface but not bonded to the substrate. The thermal pad may bespaced away from and not in direct contact with the substrate. Thethermal pad may be in direct contact with the first surface of thesubstrate.

The thermal pad may be bonded to the first surface of the substrate, anda second thermal pad may be bonded to the first LED surface and disposedbetween the thermal pad and the first LED surface. The second thermalpad may be attached and thermally coupled to the thermal pad. The secondthermal pad may be attached to the thermal pad via a thermallyconductive adhesive, thermally conductive epoxy, adhesive, anisotropicconductive adhesive, thermal grease, and/or a thermal gasket. A surfacearea of the thermal pad may be at least two times, at least five times,or at least ten times a surface area of the second thermal pad. Thethermal pad may include or consist essentially of aluminum, copper,gold, silver, silver ink, copper ink, carbon ink, and/or carbon, and/orthe second thermal pad may include or consist essentially of gold,silicon carbide, and/or aluminum nitride. The thermal pad may beelectrically isolated from at least one of the first or secondconductive traces. The thermal pad may include or consist essentially ofa material having a thermal conductivity in the range of about 150W/(m·K) to about 600 W/(m·K). The thermal pad may include or consistessentially of aluminum, copper, gold, silver, carbon, silicon, carbonink, silver ink, copper ink, silicon carbide, and/or aluminum nitride.The LED may include or consist essentially of a bare-die LED or apackaged LED.

In yet another aspect, embodiments of the invention feature a bare-dielight-emitting diode (LED) that includes or consists essentially of aplurality of semiconductor layers configured for light emission andfirst and second contacts. The plurality of semiconductor layersincludes or consists essentially of a first semiconductor layer having afirst doping polarity (i.e., n-type or p-type) and a secondsemiconductor layer having a second doping polarity opposite the firstdoping polarity. The first contact is electrically coupled to the firstsemiconductor layer and disposed on a first surface of the bare-die LED(where the bare-die LED also has a second surface opposite the firstsurface). The second contact is electrically coupled to the secondsemiconductor layer and disposed on the first surface of the bare-dieLED. The first semiconductor layer is disposed between the secondsemiconductor layer and the first surface of the bare-die LED (i.e., thesecond semiconductor layer is closer to the first surface of thebare-die LED), and a surface area of the first contact at least twotimes larger than a surface area of the second contact.

Embodiments of the invention may include one or more of the following inany of a variety of different combinations. The surface area of firstcontact may be at least five times, or even at least ten times, largerthan the surface area of the second contact. The first and secondcontact may be spaced apart from each other by a gap therebetween on thefirst surface of the LED. A midpoint of the gap may be offset from amidpoint of a length of the LED extending in a direction from the firstcontact to the second contact. The midpoint of the gap may be offsetfrom the midpoint of the length of the LED by at least 5%, or even by atleast 20%, of the length of the LED.

In an additional aspect, embodiments of the invention feature anelectronic device that includes or consists essentially of a substrate,a light-emitting diode (LED), and a thermal pad. The substrate has firstand second conductive traces on a first surface thereof. The first andsecond conductive traces are separated on the substrate by a gaptherebetween and include or consist essentially of aluminum and/orcopper. The LED has first and second distinct electrical contacts on afirst LED surface thereof. The first and second contacts are attachedand electrically coupled to, respectively, the first and secondconductive traces via solder and/or an anisotropic conductive adhesive.At least a portion of the thermal pad is disposed within the gap betweenthe first and second conductive traces. At least a portion of thethermal pad is disposed between the first surface of the substrate andthe LED. The thermal pad is electrically isolated from the first and/orsecond electrical contacts. The thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate.The thermal pad includes or consists essentially of aluminum and/orcopper. The thermal pad has a thermal conductivity greater than 150W/(m·K). A thickness of the thermal pad may be less than about 150 μm.

In another aspect, embodiments of the invention feature an electronicdevice that includes or consists essentially of a substrate, alight-emitting diode (LED), and a thermal pad. The substrate has firstand second conductive traces on a first surface thereof. The first andsecond conductive traces are separated on the substrate by a gaptherebetween and include or consist essentially of aluminum and/orcopper. The LED has first and second distinct electrical contacts on afirst LED surface thereof. The first and second contacts are attachedand electrically coupled to, respectively, the first and secondconductive traces via solder and/or an anisotropic conductive adhesive.At least a portion of the thermal pad is disposed within the gap betweenthe first and second conductive traces. At least a portion of thethermal pad is disposed between the first surface of the substrate andthe LED. The thermal pad is electrically isolated from the first and/orsecond electrical contacts. The thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate.The thermal pad may include or consist essentially of aluminum and/orcopper. The thermal pad has a thermal conductivity greater than 175W/(m·K). A thickness of the thermal pad may be less than about 150 μm.The LED may include or consist essentially of a bare-die LED or apackaged LED.

In yet another aspect, embodiments of the invention feature anelectronic device that includes or consists essentially of a substrate,a light-emitting diode (LED), a thermal pad, a backside thermal pad, andat least one thermal via. The substrate has first and second conductivetraces on a first surface thereof. The first and second conductivetraces are separated on the substrate by a gap therebetween and includeor consist essentially of aluminum and/or copper. The LED has first andsecond distinct electrical contacts on a first LED surface thereof. Thefirst and second contacts are attached and electrically coupled to,respectively, the first and second conductive traces via solder and/oran anisotropic conductive adhesive. At least a portion of the thermalpad is disposed within the gap between the first and second conductivetraces. At least a portion of the thermal pad is disposed between thefirst surface of the substrate and the LED. The thermal pad iselectrically isolated from the first and/or second electrical contacts.The thermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate. The thermal pad includesor consists essentially of aluminum and/or copper. The backside thermalpad is disposed on a second surface of the substrate opposite the firstsurface. The at least one thermal via extends from the first surface ofthe substrate to the second surface of the substrate and thermallycouples the thermal pad to the backside thermal pad. The backsidethermal pad has a thermal conductivity greater than 150 W/(m·K), and thethermal via has a thermal resistance greater than about 0.05° K/W. TheLED may include or consist essentially of a bare-die LED or a packagedLED. The backside thermal pad may include or consist essentially ofaluminum and/or copper. A thickness of the thermal pad may be less thanabout 150 μm, and/or a thickness of the backside thermal pad may be lessthan about 150 μm.

These and other objects, along with advantages and features of theinvention, will become more apparent through reference to the followingdescription, the accompanying drawings, and the claims. Furthermore, itis to be understood that the features of the various embodimentsdescribed herein are not mutually exclusive and can exist in variouscombinations and permutations. Embodiments described herein thatincorporate light-emitting diodes or other light-emitting elements maybe implemented with other electronic components instead of or inaddition to the light-emitting elements, unless otherwise indicated.Reference throughout this specification to “one example,” “an example,”“one embodiment,” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the example isincluded in at least one example of the present technology. Thus, theoccurrences of the phrases “in one example,” “in an example,” “oneembodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. As used herein, the terms “substantially,”“approximately,” and “about” mean±10%, and in some embodiments, ±5%. Theterm “consists essentially of” means excluding other materials thatcontribute to function, unless otherwise defined herein. Nonetheless,such other materials may be present, collectively or individually, intrace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic illustration of a semiconductor die in position tobecome bonded to a substrate;

FIG. 2A is a schematic cross-sectional illustration of an LEE thermalpad disposed on an LEE in accordance with various embodiments of theinvention;

FIG. 2B is a schematic plan-view illustration of an LEE thermal paddisposed on an LEE in accordance with various embodiments of theinvention;

FIG. 2C is a schematic plan-view illustration of an LEE thermal pad inaccordance with various embodiments of the invention;

FIG. 2D is a schematic plan-view illustration of an LEE thermal padhaving a width greater than an inter-trace gap spacing in accordancewith various embodiments of the invention;

FIGS. 2E and 2F are schematic cross-sectional illustrations of LEEsincorporating LEE thermal pads in accordance with various embodiments ofthe invention;

FIG. 2G is a schematic plan-view illustration of a substrate thermal paddisposed on a substrate in accordance with various embodiments of theinvention;

FIGS. 2H and 2I are, respectively, a schematic plan-view illustrationand a schematic cross-sectional illustration of a lighting deviceincorporating a substrate thermal pad disposed on a substrate, thermallycoupled to an LEE thermal pad, and located between but not in electricalcontact with conductive traces, in accordance with various embodimentsof the invention;

FIG. 3 is a schematic cross-sectional illustration of a thermal via anda substrate thermal pad in accordance with various embodiments of theinvention;

FIGS. 4A-4G are schematic illustrations of vias in accordance withvarious embodiments of the invention;

FIG. 5A is a schematic cross-sectional illustration of a lighting deviceincorporating a packaged LEE in accordance with various embodiments ofthe invention;

FIG. 5B is a schematic cross-section illustration of a lighting deviceincorporating a bare-die LEE in accordance with various embodiments ofthe invention;

FIGS. 6A and 6B are a schematic plan-view and cross-sectionalillustrations of an LEE in accordance with various embodiments of theinvention;

FIGS. 7A and 7B are schematic cross-sectional illustrations of lightingdevices incorporating LEEs in accordance with various embodiments of theinvention;

FIG. 7C is a schematic plan-view illustration of a lighting deviceincorporating an LEE in accordance with various embodiments of theinvention;

FIG. 8 is a schematic cross-sectional illustration of an LEE inaccordance with various embodiments of the invention;

FIG. 9 is a schematic plan-view illustration of an electronic devicefeaturing multiple semiconductor dies adhered to a substrate, inaccordance with various embodiments of the invention; and

FIGS. 10A and 10B are schematic cross-sectional and plan-viewillustrations, respectively, of a lighting system in accordance withvarious embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts a cross-sectional view of a portion of an assembly 100that includes a substrate (e.g., a plastic and/or flexible substrate)110 with conductive traces 120 disposed thereover. The assembly 100features an exemplary LEE 130 having multiple contacts 140. The LEE 130is positioned above conductive traces 120 prior to the die attachprocess, for use in various embodiments of the present invention. Duringthe die attach process, the contacts 140 are each electrically coupledto a conductive trace 120. As mentioned above, conventional LEDs havecontacts that provide power to the LED and that may also help extractheat from the LED. A key aspect of various embodiments of the presentinvention is the addition of an LEE thermal pad 150 to LEE 130, as shownin FIGS. 2A and 2B.

FIGS. 2A and 2B are, respectively, cross-sectional and plan views of anLEE 130 having a thermal pad 150 formed and positioned between but notin contact with the LEE contacts 140; however, this is not a limitationof the present invention, and in other embodiments the LEE thermal pad150 may be formed in other positions relative to LEE 130. The thicknessof the LEE thermal pad 150 may be less than or approximately equal tothe sum of the thicknesses of one contact 140 and the trace 120 to whichit is bonded; thus, the LEE thermal pad 150 does not significantlyinterfere with the bonding of the contacts 140 to the traces 120. In oneembodiment, the LEE thermal pad 150 may include or consist essentiallyof a thermally conductive material such as one or more metals, forexample gold, silver, copper, aluminum, or the like; however, this isnot a limitation of the present invention. In some embodiments, the LEEthermal pad 150 may include or consist essentially of one or more of thematerials such as aluminum, copper, gold, silver, carbon, silicon,polysilicon, silicon carbide, polyester, polyimide, polyethylene,silicone, polyethylene terephthalate, polycrystalline (or “poly-”) SiC,poly-AlN, SiC, AlN, and/or amorphous versions of these as well as othermaterials. In some embodiments, thermal pad 150 may have a thermalconductivity greater than about 1 W/(m·K) and preferably greater thanabout 10 W/(m·K). In some embodiments, thermal pad 150 may have athermal conductivity in the range of about 50 W/(m·K) to about 600W/(m·K) or higher.

In some embodiments, LEE thermal pad 150 may include or consistessentially of the same material as LEE contacts 140. In someembodiments, LEE thermal pad 150 may be formed in the same LEEfabrication step as LEE contacts 140. In some embodiments, LEE thermalpad 150 may have a thickness in the range of about 0.10 μm to about 20μm. In some embodiments, LEE thermal pad 150 may have a thickness in therange of about 1 μm to about 5 μm. However, this is not a limitation ofthe present invention, and in other embodiments LEE thermal pad 150includes or consist essentially a material different from LEE contact140, and/or may be formed in a process step different from the stepfabricating LEE contacts 140.

In some embodiments, LEE thermal pad 150 may have about the samethickness as contacts 140; however, this is not a limitation of thepresent invention, and in other embodiments LEE thermal pad 150 mayadvantageously have a thickness different from that of contacts 140. Insome embodiments, all or a portion of the top surface of LEE thermal pad150 and all or a portion of the top surface of LEE contacts 140 may becoplanar or substantially coplanar. For example, in some embodiments itmay be desirable for LEE thermal pad 150 to have a thickness such thatthe LEE thermal pad 150 is thermally coupled to the substrate 110 afterthe LEE 130 has been attached to the substrate 110. In anotherembodiment, the thickness of LEE thermal pad 150 may advantageously havea thickness to permit thermal coupling with a heat-dissipating devicesuch as a thermally conductive material or structure formed on substrate110.

FIG. 2B shows contacts 140 and thermal pad 150 having rectangularshapes; however, this is not a limitation of the present invention, andin other embodiments contacts 140 and/or thermal pad 150 may have othershapes, for example a square, circle or any shape. In some embodiments,contacts 140 have a different shape than thermal pad 150. In someembodiments, each contact 140 is substantially the same shape and size,while in other embodiments each contact 140 may have a different shapeand/or size. FIG. 2B shows one thermal pad 150 on LEE 130; however, thisis not a limitation of the present invention, and in other embodimentsLEE 130 may include more than one thermal pad 150.

FIG. 2C depicts an embodiment in which LEE thermal pad 150 has a widththat is less than the gap spacing 122 between the conductive traces 120.In some embodiments the LEE thermal pad 150 includes or consistsessentially of one or more electrically conductive materials (e.g.,metals), and in such embodiments the LEE thermal pad 150 preferably doesnot cause an electrical short between adjacent conductive traces 120. Insome embodiments, the LEE thermal pad 150 may overlap only oneconductive trace 120. In embodiments in which the LEE thermal pad 150 iscomposed of an electrically conductive material, the width of the LEEthermal pad 150 is generally narrow enough to prevent the LEE thermalpad 150 from overlapping both conductive traces 120 and therebyelectrically shorting the conductive traces 120.

FIG. 2D depicts another embodiment in which LEE thermal pad 150 has awidth that is greater than the gap spacing 122 between the conductivetraces 120 and is thermally (and/or electrically) coupled to only one ofthe conductive traces 120. As shown, the center of LEE 130 is offsetrelative to the gap centerline, thus preventing the LEE thermal pad 150from electrically shorting the conductive traces 120.

In some embodiments, LEE thermal pad 150 is not directly in contact withsubstrate 110, as shown in FIG. 2E, while in other embodiments LEEthermal pad 150 may be in contact with substrate 110, either directly orthrough one or more other materials, for example as shown in FIG. 2F.For example LEE thermal pad 150 may be thermally coupled to substrate110 using an adhesive, epoxy, an anisotropic conductive adhesive,thermal grease, thermal gasket material or any other material that mayaid in transfer of heat from LEE thermal pad 150 to substrate 110. Insome embodiments, the attachment medium may be engineered and/orselected to have relatively low thermal resistance and thus provideincreased heat transport from LEE thermal pad 150 to substrate 110. Insome embodiments, LEE thermal pad 150 may be thermally coupled tosubstrate 110 using the same material used to attach and electricallycouple contacts 140 to conductive traces 120.

In one embodiment, a thermal pad is formed on the substrate, identifiedas a substrate front side thermal pad 205 in FIG. 2G. As shown in FIG.2G, in some embodiments substrate front side thermal pad 205 may bespaced away from LED 130, while in other embodiments it may be incontact with LED 130.

FIGS. 2H and 2I depict plan and cross-sectional views respectively of afront side substrate thermal pad 205 disposed on the front (top) side ofsubstrate 110, thermally coupled to a LEE thermal pad 150, and locatedat least partially between but not in electrical contact with conductivetraces 120 (for clarity, the details of the structure of LEE 130 and theattachment method of LEE 130 are not shown in FIG. 2H or 2I). Front sidesubstrate thermal pad 205 may be formed into various shapes and becomposed of various materials having relatively high thermalconductivity. In one embodiment, the material selected for front sidesubstrate thermal pad 205 includes or consists essentially of thematerial used for the conductive traces 120. In some embodiments, frontside substrate thermal pad 205 may include or consist essentially of thesame material as conductive traces 120 and may be formed in the samestep as (e.g., substantially simultaneously with) conductive traces 120and may have a thickness in the range of about 0.10 μm to about 200 μm.In some embodiments, front side substrate thermal pad 205 may include orconsist essentially of the same material as conductive traces 120 andmay be formed in the same step as conductive traces 120 and may have athickness in the range of about 10 μm to about 50 μm. In anotherembodiment, the material selected for front side substrate thermal pad205 has relatively high thermal conductivity, and is thermally coupledwith LEE thermal pad 150. In another embodiment, the material selectedfor front side substrate thermal pad 205 has relatively high thermalconductivity but is electrically isolating (for example, the material offront side substrate thermal pad 205 may have an electrical resistivitygreater than approximately 100 ohm-cm, greater than approximately 1×10⁶ohm-cm, or even greater than approximately 1×10¹⁰ ohm-cm), and in theabsence of the LEE thermal pad 150, is thermally coupled with both theLEE 130 and the conductive traces 120. In another embodiment, thematerial selected for front side substrate thermal pad 205 hasrelatively high thermal conductivity but is electrically isolating, andis thermally coupled with one or both conductive traces 120 and the LEEthermal pad 150. In another embodiment, the material selected for frontside substrate thermal pad 205 is an adhesive having relatively highthermal conductivity but is electrically isolating, and is thermallycoupled with one or both conductive traces 120 and the LEE thermal pad150. In another embodiment, the material selected for front sidesubstrate thermal pad 205 is a non-adhesive material having relativelyhigh thermal conductivity but is electrically isolating, and isthermally coupled with one or both conductive traces 120 and the LEEthermal pad 150. In another embodiment, front side substrate thermal pad205 may be thermally coupled with additional heat-dissipating devices.Additional heat dissipating devices may be located on the same side ofthe substrate 110 as the LEE 130, or located on the opposite side of thesubstrate 110 and thermally coupled to the LEE thermal pad 130 or thefront side substrate thermal pad 205, as described herein. In someembodiments, substrate front side thermal pad 205 may be larger than LEEthermal pad 150; for example, in some embodiments, front side thermalpad 205 may have an area at least 2 times that of LEE thermal pad 150,at least 5 times that of LEE thermal pad 150, or even more than 10 timesthat of LEE thermal pad 150.

FIG. 3 depicts an exemplary structure 300 in accordance with embodimentsof the present invention, although alternative systems with similarfunctionality are also within the scope of the present invention. A keyaspect of various embodiments of the present invention is a thermal via220 and a back side substrate thermal pad 210, as shown in FIG. 3. Asshown, LEE 130 is electrically coupled through LEE contacts 140, 141 toconductive traces 120, 121 formed over substrate 110. In someembodiments, thermal via 220 provides a pathway for heat extractionand/or heat spreading from LEE 130 and conductive trace 121 to back sidesubstrate thermal pad 210 formed on the back side of substrate 110,i.e., the side of substrate 110 opposite the side over which is formedconductive traces 120, 121. Thus, in some embodiments the presentinvention provides a second pathway for heat removal from LEE 130. Thefirst pathway is through conductive traces 120 and 121, as shown in,e.g., FIG. 2E. The second pathway is through thermal via 220 and backside substrate thermal pad 210. The thermal via 220 may extend throughthe thickness of substrate 110, and in some embodiments of the inventionthermal via 220 may even intersect or extend partially throughconductive trace 121 and/or back side substrate thermal pad 210. WhileFIG. 3 shows LEE 130 without a LEE thermal pad 150, this is not alimitation of the present invention, and in other embodiments LEE 130may feature a thermal pad 150, as discussed herein. In some embodiments,thermal via 220 has an area in the range of about 0.1 mm² to about 10mm². In some embodiments, thermal via 220 has an area in the range ofabout 0.25 mm² to about 4 mm². In some embodiments, thermal via 220 hasa thermal conductivity of at least 1 W/(m·K). In some embodiments,thermal via 220 has a thermal conductivity of at least 10 W/(m·K). Insome embodiments, thermal via 220 may have a thermal conductivity in therange of about 50 W/(m·K) to about 600 W/(m·K) or higher.

Electrical and thermal vias may have various configurations, as depictedin FIGS. 4A-4G. Via 220 may include or consist essentially of, e.g., acrimp-type via or a through-hole that is been filled or partially filledwith electrically or thermally conductive material 2204 (FIG. 4D). Inone embodiment, a crimp-type via includes or consists essentially of aportion of conductive trace 120 electrically coupled to a portion ofback side substrate thermal pad 210, as shown in FIG. 4F. In oneembodiment, the structure in FIG. 4F may be manufactured by applyingpressure between the exposed surface of conductive trace 120 and,directly opposite it, the exposed surface of back side substrate thermalpad 210, causing substrate 110 to be deformed, resulting in a directelectrical connection between conductive trace 120 and back sidesubstrate thermal pad 210. In one embodiment, a crimp-type via includesor consists essentially of one or more conductive members that pierceand electrically couple a portion of substrate 110, a portion ofconductive trace 121 and a portion of back side substrate thermal pad210, for example as shown in FIG. 4G. While FIG. 4G shows one conductivemember forming the pierced crimp, this is not a limitation of thepresent invention and in other embodiments, more than one conductivemember may be used to form the pierced crimp connection (via). In someembodiments, a via 220 may have other configurations, for example aclamp 2201 (FIG. 4A), a rivet 2202 (FIG. 4B), a staple 2203 (FIG. 4C), awire, or the like. In some embodiments, the conductive traces 120, 121and back side substrate thermal pad 210 are formed or printed and via220 is formed as part of the forming or printing process. The means offorming thermal or electrical vias 220 between front side elements(conductive traces 120, 121, front side substrate thermal pads 205 andback side substrate thermal pad 210) is not a limitation of the presentinvention. In some embodiments, the connection between the back andfront elements may be made by folding or rolling the end of thesubstrate 110, in order to put a conductor on one side of the substratein contact with a conductor on the other side of the substrate in aregion 2205, for example as shown in FIG. 4E.

In some embodiments, conductive elements may also be formed on the backof substrate 110 for the purpose of power conduction, as described inU.S. patent application Ser. No. 13/799,807, filed on Mar. 13, 2013, theentire disclosure of which is hereby incorporated herein by reference.In such embodiments, a via similar to thermal via 220 may be used toconduct electrical current from conductive elements on one side of thesubstrate to conductive elements on the opposite side of the substrate.In some embodiments, a via may conduct electrical current and heat, asdescribed herein.

Disposed between front side elements and back side elements is substrate110. Substrate 110 may include or consist essentially of asemicrystalline or amorphous material, e.g., polyethylene naphthalate(PEN), polyethylene terephthalate (PET), polycarbonate,polyethersulfone, polyester, polyimide, polyethylene and/or paper.Substrate 110 may include multiple layers, e.g., more than one flexiblelayers or a deformable layer over a rigid layer. Depending upon thedesired application for which embodiments of the invention are utilized,substrate 110 may be substantially optically transparent, translucent,or opaque. For example, substrate 110 may exhibit a transmittance or areflectivity greater than 70% for optical wavelengths ranging betweenapproximately 400 nm and approximately 700 nm. In some embodiments,substrate 110 may exhibit a transmittance or a reflectivity of greaterthan 70% for one or more wavelengths emitted by LEE 130. In someembodiments, substrate 110 may be substantially insulating, and may havean electrical resistivity greater than approximately 100 ohm-cm, greaterthan approximately 1×10⁶ ohm-cm, or even greater than approximately1×10¹⁰ ohm-cm. In some embodiments, substrate 110 may have a relativelyhigh thermal conductivity. For example where substrate 110 includes orconsists essentially of PET, the thermal conductivity may be in therange of about 0.10 to about 0.03 W m⁻¹ K⁻¹. In some embodiments,substrate 110 may have a thickness in the range of about 20 μm to about1000 μm.

Conductive traces 120, 121, front side substrate thermal pad 205, andback side substrate thermal pad 210 may be formed via conventionaldeposition, photolithography, and etching processes, plating processes,lamination, lamination and patterning, evaporation sputtering, or thelike, or may be formed using a variety of different printing processes.For example, traces 120, 121, front side substrate thermal pad 205, andback side substrate thermal pad 210 may be formed via screen printing,flexographic printing, ink-jet printing, and/or gravure printing.Conductive traces 120, 121, front side substrate thermal pad 205, andback side substrate thermal pad 210 may include or consist essentiallyof a conductive material (e.g., an ink or a metal, metal film, or otherconductive materials, or the like), which may include one or moreelements such as silver, gold, aluminum, chromium, copper, silver ink,copper ink, carbon ink, and/or carbon. In one embodiment, conductivetraces 120 include or consist essentially of copper, for example toenable a solder process of die attach, while LEE thermal pad 150includes or consists essentially of gold. In one embodiment, conductivetraces 120 include or consist essentially of aluminum.

In some embodiments, conductive traces 120, 121, front side substratethermal pad 205, and back side substrate thermal pad 210 may have athickness in the range of about 50 nm to about 1000 μm. In someembodiments, conductive traces 120, 121, front side substrate thermalpad 205, and back side substrate thermal pad 210 may have a thickness inthe range of about 10 μm to about 150 μm, as such small thicknesses maysubstantially retain the flexibility of flexible substrates 110 whileadvantageously providing heat dissipation. In some embodiments,conductive traces 120, 121, front side substrate thermal pad 205, andback side substrate thermal pad 210 may have a maximum thickness ofabout 150 μm, in order to retain substrate flexibility. In someembodiments, the thickness of traces 120, 121 may be determined by thecurrent to be carried thereby. While the thickness of one or more oftraces 120, 121, front side substrate thermal pad 205, and back sidesubstrate thermal pad 210 may vary, the thickness is generallysubstantially uniform along the length of the trace to simplifyprocessing. However, this is not a limitation of the present inventionand in other embodiments the thickness and/or material of traces 120,121, front side substrate thermal pad 205, and back side substratethermal pad 210 may vary. In some embodiments, all or a portion ofconductive traces 120, 121, front side substrate thermal pad 205 andback side substrate thermal pad 210 may be covered or encapsulated. Insome embodiments, a layer of material, for example electricallyinsulating material, may be formed over all or a portion of traces 120,121, front side substrate thermal pad 205, and back side substratethermal pad 210. Such a material may include, e.g., a sheet of materialsuch as used for substrate 110, a printed layer, for example usingscreen, ink jet, stencil or other printing means, a laminated layer, orthe like. Such a printed layer may include, for example, an ink, aplastic and oxide, or the like. The covering material and/or the methodby which it is applied are not a limitation of the present invention.

In some embodiments, front side substrate thermal pad 205 and back sidesubstrate thermal pad 210 may include or consist essentially of the samematerial as conductive traces 120, 121; however, this is not alimitation of the present invention, and in other embodiments front sidesubstrate thermal pad 205 and back side substrate thermal pad 210include or consist essentially of a material different from conductivetraces 120, 121.

In some embodiments, front side substrate thermal pad 205 and back sidesubstrate thermal pad 210 may have about the same thickness asconductive traces 120, 121; however, this is not a limitation of thepresent invention, and in other embodiments front side substrate thermalpad 205 and back side substrate thermal pad 210 may advantageously havea thickness different from that of conductive traces 120, 121. Forexample, in some embodiments it may be desirable to have a certain trace120, 121 thickness and/or material for bonding of an LEE to conductivetraces 120, 121, while having a different material and or thickness forfront side substrate thermal pad 205 and back side substrate thermal pad210.

LEE 130 may be electrically coupled to conductive traces 120, 121 and/ormechanically coupled to conductive traces 120, 121 and/or substrate 110using a variety of means, for example soldering, wire bonding,conductive adhesive, anisotropic conductive adhesive (ACA) or the like.The method of electrical and/or mechanical coupling or bonding is not alimitation of the present invention. However, the method of attachmentmay impact the flow of heat out of LEE 130.

FIG. 5A shows one embodiment of the present invention in which LEE 130is electrically coupled and mechanically attached to conductive traces120, 121 using a bonding material 375. In some embodiments, bondingmaterial 375 includes or consists essentially of a solder. In someembodiments, bonding material 375 includes or consists essentially of aconductive adhesive, e.g., an isotropic conductive adhesive or an ACA.FIG. 5A also shows an embodiment in which back side substrate thermalpad 210 is relatively thicker than conductive trace 121. In FIG. 5A, LEE130 includes or consists essentially of a packaged LEE 130 (i.e., a bareLEE die disposed within a package, where contacts 140, 141 are disposedand/or extend out of the package and are electrically coupled to the diewithin the package); however this is not a limitation of the presentinvention, and in other embodiments LEE 130 may include or consistessentially of a bare semiconductor die LEE 130, as shown in FIG. 5B. Insome embodiments, back side substrate thermal pad 210 has a thickness inthe range of about 0.5 μm to about 200 μm. In some embodiments, backside substrate thermal pad 210 has a thickness in the range of about 10μm to about 100 μm.

In some embodiments, thermal via 220 may have an area (e.g., in across-section substantially parallel to a surface of substrate 110and/or conductive trace 120) in the range of about 0.0001 cm² to about 1cm². In some embodiments, thermal via 220 may have a thicknesssubstantially the same as that of substrate 110. In some embodiments,thermal via 220 may have a thickness in the range of about 10 μm toabout 500 μm.

In some embodiments, thermal via 220 may have a thermal resistance inthe range of about 0.05° K/W to about 10° K/W. In one embodiment,substrate 110 has a thickness of about 38 μm, thermal via 220 has anarea of about 0.01 cm², and thermal via includes or consists essentiallyof aluminum having a thermal conductivity of about 235 W/(m·K),resulting in the thermal via 220 having a thermal resistance of about1.62° K/W. In one embodiment, substrate 110 has a thickness of about 38μm, thermal via 220 has an area of about 0.04 cm², and thermal viaincludes or consists essentially of aluminum having a thermalconductivity of about 235 W/(m·K), resulting in the thermal via having athermal resistance of about 0.4° K/W. In one embodiment, substrate 110has a thickness of about 38 μm, thermal via 220 has an area of about0.04 cm², and thermal via includes or consists essentially of silverhaving a thermal conductivity of about 406 W/(m·K), resulting in thethermal via having a thermal resistance of about 0.23° K/W. In oneembodiment, substrate 110 has a thickness of about 125 μm, thermal via220 has an area of about 0.01 cm² and thermal via includes or consistsessentially of aluminum having a thermal conductivity of about 235W/(m·K), resulting in the thermal via having a thermal resistance ofabout 5.3° K/W.

FIG. 5B shows one embodiment of the present invention in which LEE 130includes or consists essentially of a bare semiconductor die. In thisembodiment, LEE 130 is electrically coupled to conductive traces 120,121 and mechanically attached to conductive traces 120, 121 and aportion of substrate 110 using a conductive adhesive 570. In someembodiments conductive adhesive 570 includes or consists essentially ofan ACA. In some embodiments, ACA 570 includes or consists essentially ofan adhesive component 572 in which is infused conductive particles 571.ACA 570 enables electrical interconnection in one direction (e.g.,vertically between a device contacts 140, 141 and conductive traces 120,121, respectively) through conductive particles 571, but prevents it inother directions (e.g., horizontally between contacts 140, 141 on LEE130 or between conductive traces 120, 121 on a substrate 110). VariousACAs are pressure-activated, and in some embodiments they may requireprovision of “stud bumps” or other metallic projections on the surfaceto which the LEE 130 is to be bonded or on the LEE contacts 140, 141 inorder to create the anisotropic electrical conductivity and promoteadhesion; however, this is not a limitation of the present invention andin other embodiments stud bumps need not be required, as described inthe '121 patent. In some embodiments, conductive particles 571 includeor consist essentially of spheres of a conductive material or of aninsulating material coated with a conductive material (such as metal ora conductive material coated with an insulating material). In someembodiments, conductive particles 571 may be substantially spherical,while in other embodiments conductive particles 571 may have othershapes, for example a more planar flake shape or a three-dimensionalflake shape. In some embodiments, pressure and heat are applied toactivate and cure the ACA. FIG. 5B depicts the use of pressure-activatedACA 570 to connect a LEE 130 to a substrate 110. As shown, the LEE 130having multiple contacts 140, 141 has been adhered and electricallyconnected to a substrate 110 via the ACA 570. As shown, the conductiveparticles 571 provide electrical connectivity between each contact 140,141 and its respective trace 120, 121, but are dispersed at asufficiently low density such that an electrical connection is notformed between the contacts 140, 141 and/or the traces 120, 121.

In other embodiments LEE 130 may be electrically coupled andmechanically attached to conductive traces 120, 121 and optionallysubstrate 110 using a conductive adhesive (e.g., an isotropic conductiveadhesive) or a combination of conductive and non-conductive adhesives.

Referring to FIG. 5B, in some embodiments LEE 130 includes a substrate510 with one or more semiconductor layers 530, 540, 550 disposedthereover. LEE 130 may also include an optional insulating layer 520and/or an optional mirror layer 590. Contacts 140 and 141 make contactto semiconductor layers 530 and 540 respectively. In this exemplaryembodiment, LEE 130 represents a light-emitting device such as a LED ora laser, but other embodiments of the invention feature one or moresemiconductor dies with different or additional functionality, e.g.,processors, sensors, detectors, and the like. Non-LED dies may or maynot be bonded as described herein, and may have contact geometriesdiffering from those of the LEDs; moreover, they may or may not havesemiconductor layers disposed over a flexible substrate as discussedbelow.

Referring to FIG. 5B, in one embodiment LEE 130 includes a firstsemiconductor layer 530, an active layer 550, and a second semiconductorlayer 540. In this example, the first semiconductor layer 530 may be ann-type semiconductor layer, and the second semiconductor layer 540 maybe a p-type semiconductor layer. The first semiconductor layer 530 maybe configured to supply electrons to the active layer 550, and thesecond semiconductor layer 540 may be configured to supply holes to theactive layer 550. Electrons and holes may be coupled in the active layer550, and light may be emitted from the active layer 550 as a result ofor in response to the coupling. However, this embodiment is not alimitation of the present invention, and in other embodiments thesemiconductor layers may be interchanged. In this embodiment, contact140 is electrically coupled to the n-type semiconductor layer 530 whilecontact 141 is electrically coupled to the p-type semiconductor layer540. However, this is not a limitation of the present invention, and inother embodiments the connections may be reversed. FIG. 5B shows onecontact each to the p-type and n-type semiconductor layers; however,this is not a limitation of the present invention, and other embodimentsmay have more than one contact to either or both semiconductor layers.

As shown in FIG. 5B, in preferred embodiments LEE 130 is patterned andetched (e.g., via conventional photolithography and etch processes) suchthat a portion of layer 530 is exposed in order to facilitate electricalcontact to layer 530 and layer 540 on the same side of LEE 130 (andwithout, for example, the need to make contact to layer 530 throughsubstrate 510 or to make contact to layer 530 with a shunt electricallyconnecting a contact pad over layer 540 to layer 530). One or moreportions of layers 540, 550 are removed (or never formed) in order toexpose a portion of layer 530. In order to facilitate electrical contactto LEE 130, discrete electrical contacts 140, 141 are formed on layers530, 540, respectively. Electrical contacts 140, 141 may each include orconsist essentially of a suitable conductive material, e.g., one or moremetals or metal alloys conductive oxides, or other suitable conductivematerials. While FIG. 5B shows contacts 140 and 141 as non-coplanar,this is not a limitation of the present invention and in otherembodiments contacts 140 and 141 (at least the outer surfaces thereof)are coplanar or substantially coplanar.

Substrate 510 may include or consist essentially of one or moresemiconductor materials, e.g., silicon, GaAs, InP, GaN, and may be dopedor substantially undoped (e.g., not intentionally doped). In someembodiments, substrate 510 includes or consists essentially of sapphireor silicon carbide. Substrate 510 may be substantially transparent to awavelength of light emitted by the LEE 130. As shown for alight-emitting device, semiconductor layers 530, 540, 550 may includefirst and second doped layers 530, 540, which preferably are doped withopposite polarities (i.e., one n-type doped and the other p-type doped).One or more light-emitting layers 550, e.g., one or more quantum wells,may be disposed between layers 530, 540. Each of layers 530, 540, 550may include or consist essentially of one or more semiconductormaterials, e.g., silicon, InAs, AlAs, GaAs, InP, AlP, GaP, InSb, GaSb,AlSb, GaN, AlN, InN, and/or mixtures and alloys (e.g., ternary orquaternary, etc. alloys) thereof. In preferred embodiments, LEE 130 isan inorganic, rather than a polymeric or organic, device. As referred toherein, LEEs may be packaged or unpackaged unless specifically indicated(e.g., a bare-die LEE is an unpackaged semiconductor die). In someembodiments, substantially all or a portion of substrate 510 is removedprior to or after the bonding of LEE 130 described below. Such removalmay be performed by, e.g., chemical etching, laser lift-off, mechanicalgrinding and/or chemical-mechanical polishing or the like. In someembodiments, all or a portion of substrate 510 may be removed and asecond substrate—e.g., one that is transparent to or reflective of awavelength of light emitted by LEE 130—is attached to substrate 510 orsemiconductor layers 520 prior to or after the bonding of LEE 130 asdescribed below. In some embodiments, substrate 510 includes or consistsessentially of silicon, and all or a portion of silicon substrate 510may be removed prior to or after the bonding of LEE 130 described below.Such removal may be performed by, e.g., chemical etching, laser liftoff, mechanical grinding and/or chemical-mechanical polishing or thelike.

In preferred embodiments the conductive trace 120, 121 coupled to thethermal via 220 is coupled to the contact 140, 141 of LEE 130 that hasthe least thermal resistance to the heat-generating region in LEE 130.For example, FIG. 5B shows contact 141, which is attached to the p-typesemiconductor layer 540, being coupled to thermal via 220. In someembodiments, the p-type semiconductor layer 540 is closest to thesurface of LEE 130 and contact 141 provides the lowest thermalresistance to conductive trace 121. However, this is not a limitation ofthe present invention, and in other embodiments either contact may bethermally coupled to thermal via 220. While FIG. 5B shows a bare-die LEE130, ACA 570 may also be used with packaged LEEs 130.

The size and shape of contacts 140, 141 of LEE 130 may be designed toreduce thermal resistance between LEE 130 and heat-dissipating elements,including, for example, conductive traces 120, 121, front side substratethermal pads 205, thermal via 220, and back side substrate thermal pad210.

In one embodiment of the present invention, an asymmetric contact designmay be used to enhance heat transfer out of LEE 130. FIGS. 6A and 6Brespectively depict a plan view and cross-sectional view of an LEE 130that features asymmetric contact sizes. In FIGS. 6A and 6B, p-contact141 is relatively larger than n-contact 140; however, this is not alimitation of the present invention, and in other embodiments, p-contact141 is relatively smaller than the n-contact 140. In some embodiments, aspacing 610 between contact 140 and 141 may be advantageouslycontrolled. For example, in embodiments in which LEE 130 is electricallycoupled using ACA it may be desirable to set a minimum dimension 610,for example to be at least as large as gap width 122 as shown in FIG.2C. In some embodiments, it may be desirable to maintain a certain diearea while maximizing gap width 122. In such embodiments, die width 620may be reduced while die length 630 is increased. FIG. 6A shows contact140 having a width 640 and a length 650 and contact 141 having a width670 and a length 660. FIG. 6A shows width 670 as equal or substantiallyequal to width 640; however, this is not a limitation of the presentinvention, and in other embodiments width 670 may not be the same aswidth 640. Table 1 lists dimensions in microns for several exemplaryembodiments of LEE 130 of FIG. 6A. These dimensions are exemplary andare not limitations of the present invention.

TABLE 1 Dimension Example 630 620 640 650 670 660 1 300 200 180 30 18030 2 300 200 180 30 180 70 3 600 200 180 50 180 250 4 600 200 180 100180 250

In some embodiments the area of one contact, for example contact 141,may be in the range of about 1.25 times to about 10 times larger thanthe area of the other contact, here contact 140. In some embodiments,the area of one contact, for example contact 141, may be in the range ofabout 2 times to about 5 times larger than the area of the othercontact, here contact 140.

In some embodiments, the width of conductive trace 121 is approximatelyequal to the width of contact 141 or larger, to enhance thermalconduction from the LEE 130. In another embodiment, conductive trace 121may be narrower than the width of contact 141. In some embodiments, LEE130 includes an asymmetrically larger contact 141 placed in an offsetposition on the conductive traces 121, to increase the area for heattransfer from LEE 130 to conductive trace 121. In another embodiment,the gap between the conductive traces 120, 121 is minimized to permitrelatively larger contact area between contacts 140, 141 and conductivetraces 120, 121. FIG. 6B depicts a cross-sectional view of LEE 130having asymmetric p-contact 141, disposed over conductive trace 121 inan offset configuration. In some embodiments, the center of spacing 610may be offset from the center of LEE 130 by an amount up to about 5% ofa length 630 of LEE 130. In some embodiments, the center of spacing 610may be offset from the center of LEE 130 by an amount up to about 20% ofthe length 630 of LEE 130. In some embodiments, the center of spacing610 may be offset from the center of LEE 130 by an amount up to about40% of the length 630 of LEE 130.

While FIGS. 3, 5A, and 5B show one thermal via 220 per back sidesubstrate thermal pad 210, this is not a limitation of the presentinvention, and in other embodiments a back side substrate thermal pad210 may be associated with more than one thermal via. Referring to FIG.7A, some embodiments may feature multiple thermal vias and multiplesubstrate thermal pads. For example, FIG. 7A shows an embodiment withseparate thermal vias 220, 220′ and substrate thermal pads 210, 210′ foreach contact 120, 121 respectively (for clarity, the details of thestructure of LEE 130 and the attachment method of LEE 130 are not shownin FIG. 7A). As shown in FIG. 7A, thermal vias 220 and 220′ are notelectrically coupled to each other, nor are substrate thermal pads 210and 210′ electrically coupled to each other.

Some embodiments may include more than two substrate thermal pads 210(one or more being disposed on each of the front or back or both sidesof the substrate 110) coupled with the contacts 120, 121 and LEE thermalpad 150. For example, FIGS. 7B and 7C show a structure with three backsubstrate thermal pads and one front substrate thermal pad. FIGS. 7B and7C respectively show cross-sectional and plan views of an embodimentwith separate thermal vias 220, 220′ and back side substrate thermalpads 210′, 210 for each contact 120, 121 respectively, and separatethermal vias 220″, 220′″ coupling front side substrate thermal pad 205with back side substrate thermal pad 210″. In one embodiment, conductivetraces 120, 121 include or consist essentially of the same material asfront side substrate thermal pad 205 and back side substrate thermalpads 210, 210′, and 210″. However, this is not a limitation of thepresent invention, and in other embodiments conductive traces 120, 121,front side substrate thermal pad 205 and back side substrate thermalpads 210, 210′, and 210″ may include or consist essentially of differentmaterials. In one embodiment, conductive traces 120, 121 include orconsist essentially of copper, for example to enable use of solder toattach contacts 140, 141, while back side substrate thermal pad 210 (andif present 210′ and 210″) includes or consists essentially of aluminum.

Elements of the LEE 130 other than the contacts 140, 141 may also beutilized for heat removal. A reflector 890 shown in FIG. 8 may bedisposed between or above portions of contacts 140, 141 and overportions or substantially all of layer 540. In some embodimentsreflector 890 is reflective to at least some or all wavelengths of lightemitted by LEE 130 and may include or consist essentially of any ofvarious materials. In one embodiment, reflector 890 is non-conductive(i.e., electrically insulating). In some embodiments, reflector 890 maybe a Bragg reflector. In some embodiments, reflector 890 may include orconsist essentially of one or more conductive materials, e.g., metalssuch as silver, gold, platinum, etc. Instead of or in addition toreflector 890, exposed surfaces of LEE 130 except for contacts 140, 141may optionally be coated with one or more layers of an electricallyinsulating material, e.g., a nitride such as silicon nitride or an oxidesuch as silicon dioxide. In some embodiments, contacts 140, 141 includea bond portion, e.g. a bond pad, for connection to conductive traces120, 121 and a current-spreading portion for providing more uniformcurrent through LEE 130, and in some embodiments, one or more layers ofan insulating material are formed over all or portions of LEE 130 exceptfor the bond portions of contacts 140, 141. FIG. 5B shows a schematic ofLEE 130 with insulating material 520 covering substantially the entiresurface of LEE 130 except for contacts 140, 141. Insulating material 520may include or consist essentially of, for example, polyimide, siliconnitride, silicon oxide and/or silicon dioxide. Such insulating material520 may cover all or portions of the top and sides of LEE 130 as well asportions of the top and sides of layers 530, 540, and 550. Insulatingmaterial 520 may act to prevent shorting between reflector 890 and/orportions of semiconductor layers 530, 540, and 550 and conductive traces120, 121, or both during and/or after the bonding of the contacts to theconductive traces.

In some embodiments, reflector 890 of LEE 130 may be used as aheat-dissipating device. In some embodiments, it may be advantageous toeliminate the LEE thermal pad 150 and instead use an electricallynon-conductive material having a relatively high thermal conductivity toconduct heat from the reflector 890 to heat-dissipating elements such asthe conductive traces 120, 121, the front side substrate thermal pad205, or the substrate 110 itself or to ambient. In other embodiments,the reflector 890 may optionally be covered by a thermally conductiveoxide layer.

FIG. 9 depicts an electronic device 900 featuring an array of LEEs 130each adhered between pairs of conductive traces 120 as described above.As shown, electronic device 900 includes three serially-connectedstrings 910 of LEEs 130. Electronic device 900 also includes circuitry920 electrically connected to one or more of the strings 910. Thecircuitry 920 may include or consist essentially of portions of (in thecase, for example, of a distributed power supply/driver) orsubstantially all of drive circuitry, sensors, control circuitry,dimming circuitry, and or power-supply circuitry or the like, and mayalso be adhered (e.g., via an adhesive) or otherwise attached tosubstrate 110. Circuitry 920 may even be disposed on a circuit board(e.g., a printed circuit board) that itself may be mechanically and/orelectrically attached to substrate 110. In other embodiments, circuitry920 is separate from substrate 110. While FIG. 9 depicts the LEE 130serially connected in strings 910, and strings 910 connected orconnectable in parallel, other LEE-interconnection schemes are possibleand within the scope of the invention.

In some embodiments, electronic device 900 may be fabricated on aflexible substrate 110. In some embodiments, electronic device may beformed in a roll-to-roll process, in which a sheet of the flexiblesubstrate material travels through different processing stations. Suchroll-to-roll processing may, for example, include the formation ofconductive traces 120, dispensing of the adhesive or ACA, the placementof LEEs 130, and the curing of the adhesive, as well as for the bondingof any additional components and/or formation of one or more phosphormaterials at least partially surrounding LEEs 130. In addition,electronic device 900 may also include other passive and/or activeelectronic devices attached to substrate 110, including, e.g., sensors,antennas, resistors, inductors, capacitors, thin-film batteries,transistors and/or integrated circuits. Such other passive and/or activeelectronic devices may be electrically coupled to conductive traces 120or LEEs 130 with an adhesive, an ACA or by other means. Roll-to-rollprocessing may also use other means for attachment and electricalcoupling of LEE 130 and/or other components, for example solder or wirebonding. The means for attachment and electrical coupling of LEE 130and/or other components is not a limitation of the present invention.

Various embodiments of the present invention utilize electricallyconductive elements on the back of an electrical device, or lightsheet,1000 to enhance thermal conduction away from LEEs 130, as shown in FIGS.10A and 10B. In some embodiments, the lightsheet 1000 may include one ormore conductive elements on the back of the substrate, for example onthe back of substrate 110 as illustrated in FIGS. 10A and 10B. Suchconductive elements, which may also be called power conductors, backpower conductors, power bus, power bus lines, and/or bus lines, may beconfigured and used to permit much longer lightsheets while stillmaintaining acceptable voltage loss in the power conductor and thusacceptably high efficiency. FIGS. 10A and 10B depict an electricaldevice 1000 in cross-section and plan view respectively. Electricaldevice 1000 includes power conductors 930, 940, conductive traces 120,and LEEs 130 formed over substrate 110. Electrical device 1000 alsofeatures back power conductors 1010, 1020 that are formed on the side ofsubstrate 110 opposite the side on which power conductors 930, 940 areformed. Back power conductors 1010, 1020 may be electrically andthermally coupled to power conductors 930, 940 respectively, using oneor more vias 220.

Furthermore, one or more LEEs 130 may be bonded to traces 120 on theback side of substrate 110 in a similar or different fashion to thatdepicted in FIG. 10A, and/or multiple substrates 110 having LEEs 130 andtraces 120 thereon may be stacked to form multi-layer devices. Referringto FIG. 10B, lightsheet 1000 features an array of LEEs 130 eachelectrically coupled between conductive traces 120, and power conductors930, 940 providing power to conductive traces 120, all of which aredisposed over a substrate 110. In one embodiment, the power conductors930, 940 may be formed on the one side of the substrate 110 andelectrically coupled by means of vias 220 to the conductive traces 120on the opposite side of the substrate 110. Where vias 220 areelectrically coupled to LEEs 130 and power conductors 930, 940, improvedthermal conduction of heat away from the LEE 130 may occur.

In various embodiments, disposed over and at least partially surroundingthe LEE 130 is a phosphor material for converting at least a portion oflight emitted by the LEE 130 to light of a different wavelength. In someembodiments, the LEE 130 having a wavelength-conversion materialgenerates white light by combining the short-wavelength radiant flux(e.g., blue light) emitted by the LEE with long-wavelength radiant flux(e.g., yellow light) emitted by the wavelength-conversion material. Thechromaticity (or color), color temperature, and color-rendering indexare determined by the relative intensities of the component colors. Forexample, the light color may be adjusted from “warm white” with acorrelated color temperature (CCT) of 2700 Kelvin or lower to “coolwhite” with a CCT of 6500 Kelvin or greater by varying the type oramount of phosphor material. White light may also be generated solely orsubstantially only by the light emitted by the one or morewavelength-conversion materials.

As used herein, “phosphor” refers to any material that shifts thewavelengths of light irradiating it and/or that is fluorescent and/orphosphorescent. As used herein, a “phosphor” may refer to only thepowder or particles (of one or more different types) or to the powder orparticles with the binder, and in some circumstances may refer toregion(s) containing only the binder (for example, in a remote-phosphorconfiguration in which the phosphor is spaced away from the LEE). Theterms “wavelength-conversion material” and “light-conversion material”are utilized interchangeably with “phosphor” herein. Thelight-conversion material is incorporated to shift one or morewavelengths of at least a portion of the light emitted by LEEs to other(i.e., different) desired wavelengths (which are then emitted from thelarger device alone or color-mixed with another portion of the originallight emitted by the LEE). A light-conversion material may include orconsist essentially of phosphor powders, quantum dots, organic dyes, orthe like within a transparent binder. Phosphors are typically availablein the form of powders or particles, and in such case may be mixed inbinders. An exemplary binder is silicone, i.e., polyorganosiloxane,which is most commonly polydimethylsiloxane (PDMS). Phosphors vary incomposition, and may include lutetium aluminum garnet (LuAG or GAL),yttrium aluminum garnet (YAG) or other phosphors known in the art. GAL,LuAG, YAG and other materials may be doped with various materialsincluding for example Ce, Eu, etc. The specific components and/orformulation of the phosphor and/or matrix material are not limitationsof the present invention.

The binder may also be referred to as an encapsulant or a matrixmaterial. In one embodiment, the binder includes or consists essentiallyof a transparent material, for example silicone-based materials orepoxy, having an index of refraction greater than 1.35. In oneembodiment the binder and/or phosphor includes or consists essentiallyof other materials, for example fumed silica or alumina, to achieveother properties, for example to scatter light, or to reduce settling ofthe powder in the binder. An example of the binder material includesmaterials from the ASP series of silicone phenyls manufactured by ShinEtsu, or the Sylgard series manufactured by Dow Corning.

While the discussion herein has mentioned blue LEEs and phosphors, thatwhen combined produce white light, the concepts may be used with respectto LEEs emitting at any wavelength and phosphors orwavelength-conversion materials with any emission wavelengths that mayin combination or alone be used to produce any color.

In general in the above discussion the arrays of semiconductor dies,light emitting elements, optics, and the like have been shown as squareor rectangular arrays; however this is not a limitation of the presentinvention and in other embodiments these elements may be formed in othertypes of arrays, for example hexagonal, triangular or any arbitraryarray. In some embodiments these elements may be grouped into differenttypes of arrays on a single substrate.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. An electronic device comprising: a substratehaving first and second conductive traces on a first surface thereof,the first and second conductive traces being separated on the substrateby a gap therebetween; a light-emitting diode (LED) having first andsecond distinct electrical contacts on a first LED surface thereof, thefirst and second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; and a thermal pad,wherein (i) at least a portion of the thermal pad is disposed within thegap between the first and second conductive traces, (ii) at least aportion of the thermal pad is disposed between the first surface of thesubstrate and the LED, (iii) the thermal pad is electrically isolatedfrom at least one of the first or second electrical contacts, (iv) thethermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate, (v) the thermal pad hasa thermal conductivity greater than 1 W/(m·K), and (vi) the thermal padcomprises one or more electrically conductive materials.
 2. Anelectronic device comprising: a substrate having first and secondconductive traces on a first surface thereof, the first and secondconductive traces being separated on the substrate by a gaptherebetween; a light-emitting diode (LED) having first and seconddistinct electrical contacts on a first LED surface thereof, the firstand second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; and a thermal pad,wherein (i) at least a portion of the thermal pad is disposed within thegap between the first and second conductive traces, (ii) at least aportion of the thermal pad is disposed between the first surface of thesubstrate and the LED, (iii) the thermal pad is electrically isolatedfrom at least one of the first or second electrical contacts, (iv) thethermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate, (v) the thermal pad hasa thermal conductivity greater than 1 W/(m·K), and (vi) the thermal padis partially disposed over only one of the first or second conductivetraces but not the other.
 3. An electronic device comprising: asubstrate having first and second conductive traces on a first surfacethereof, the first and second conductive traces being separated on thesubstrate by a gap therebetween; a light-emitting diode (LED) havingfirst and second distinct electrical contacts on a first LED surfacethereof, the first and second contacts being attached and electricallycoupled to, respectively, the first and second conductive traces; and athermal pad, wherein (i) at least a portion of the thermal pad isdisposed within the gap between the first and second conductive traces,(ii) at least a portion of the thermal pad is disposed between the firstsurface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) a surface area of the thermal pad is greater than at least oneof a surface area of the first electrical contact or a surface area ofthe second electrical contact.
 4. An electronic device comprising: asubstrate having first and second conductive traces on a first surfacethereof, the first and second conductive traces being separated on thesubstrate by a gap therebetween; a light-emitting diode (LED) havingfirst and second distinct electrical contacts on a first LED surfacethereof, the first and second contacts being attached and electricallycoupled to, respectively, the first and second conductive traces; and athermal pad, wherein (i) at least a portion of the thermal pad isdisposed within the gap between the first and second conductive traces,(ii) at least a portion of the thermal pad is disposed between the firstsurface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) the thermal pad is bonded to the first surface of the substrateand to the first LED surface.
 5. An electronic device comprising: asubstrate having first and second conductive traces on a first surfacethereof, the first and second conductive traces being separated on thesubstrate by a gap therebetween; a light-emitting diode (LED) havingfirst and second distinct electrical contacts on a first LED surfacethereof, the first and second contacts being attached and electricallycoupled to, respectively, the first and second conductive traces; and athermal pad, wherein (i) at least a portion of the thermal pad isdisposed within the gap between the first and second conductive traces,(ii) at least a portion of the thermal pad is disposed between the firstsurface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) the thermal pad is bonded to only one of the first surface ofthe substrate or the first LED surface, but not both.
 6. The electronicdevice of claim 5, wherein (i) the thermal pad is bonded to the firstLED surface but not bonded to the substrate, and (ii) the thermal pad isspaced away from and not in direct contact with the substrate.
 7. Theelectronic device of claim 5, wherein (i) the thermal pad is bonded tothe first LED surface but not bonded to the substrate, and (ii) thethermal pad is in direct contact with the first surface of thesubstrate.
 8. An electronic device comprising: a substrate having firstand second conductive traces on a first surface thereof, the first andsecond conductive traces being separated on the substrate by a gaptherebetween; a light-emitting diode (LED) having first and seconddistinct electrical contacts on a first LED surface thereof, the firstand second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; and a thermal pad,wherein (i) at least a portion of the thermal pad is disposed within thegap between the first and second conductive traces, (ii) at least aportion of the thermal pad is disposed between the first surface of thesubstrate and the LED, (iii) the thermal pad is electrically isolatedfrom at least one of the first or second electrical contacts, (iv) thethermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate, (v) the thermal pad hasa thermal conductivity greater than 1 W/(m·K), and (vi) the thermal padis bonded to at least one of the first surface of the substrate or thefirst LED surface via at least one of a thermally conductive adhesive,thermally conductive epoxy, adhesive, anisotropic conductive adhesive,solder, thermal grease, or a thermal gasket.
 9. The electronic device ofclaim 1, wherein the thermal pad is electrically isolated from at leastone of the first or second conductive traces.
 10. The electronic deviceof claim 1, wherein the thermal pad comprises a material having athermal conductivity in the range of about 75 W/(m·K) to about 600W/(m·K).
 11. An electronic device comprising: a substrate having firstand second conductive traces on a first surface thereof, the first andsecond conductive traces being separated on the substrate by a gaptherebetween; a light-emitting diode (LED) having first and seconddistinct electrical contacts on a first LED surface thereof, the firstand second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; and a thermal pad,wherein (i) at least a portion of the thermal pad is disposed within thegap between the first and second conductive traces, (ii) at least aportion of the thermal pad is disposed between the first surface of thesubstrate and the LED, (iii) the thermal pad is electrically isolatedfrom at least one of the first or second electrical contacts, (iv) thethermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate, (v) the thermal pad hasa thermal conductivity greater than 1 W/(m·K), and (vi) the thermal padcomprises at least one of aluminum, copper, gold, silver, silver ink,copper ink, carbon ink, silicon carbide, or aluminum nitride.
 12. Anelectronic device comprising: a substrate having first and secondconductive traces on a first surface thereof, the first and secondconductive traces being separated on the substrate by a gaptherebetween; a light-emitting diode (LED) having first and seconddistinct electrical contacts on a first LED surface thereof, the firstand second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; a thermal pad; abackside thermal pad disposed on a second surface of the substrateopposite the first surface; and at least one thermal via (a) extendingfrom the first surface of the substrate to the second surface of thesubstrate and (b) thermally coupling the thermal pad to the backsidethermal pad, wherein (i) at least a portion of the thermal pad isdisposed within the gap between the first and second conductive traces,(ii) at least a portion of the thermal pad is disposed between the firstsurface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) the backside thermal pad has a thermal conductivity greaterthan 75 W/(m·K).
 13. The electronic device of claim 12, wherein the atleast one thermal via is bonded to the thermal pad and to the backsidethermal pad.
 14. The electronic device of claim 12, wherein each said atleast one thermal via has a thermal resistance in the range of about0.05 ° K/W to about 10° K/W.
 15. The electronic device of claim 12,wherein the at least one thermal via comprises at least one of (i) acrimp-type via between the first and second surfaces of the substrate,(ii) a staple extending through the substrate in at least two discretelocations, (iii) a rivet extending through the substrate, (iv) a clampextending around the substrate but not through the substrate, or (v) awire.
 16. The electronic device of claim 12, wherein the at least onethermal via comprises a through-hole extending through the substrate andsubstantially filled with a thermally conductive material having athermal conductivity greater than 10 W/(m·K).
 17. The electronic deviceof claim 1, wherein the substrate is flexible.
 18. The electronic deviceof claim 1, wherein the substrate comprises at least one of polyethylenenaphthalate, polyethylene terephthalate, epoxy, polycarbonate,polyethersulfone, polyester, polyimide, polyethylene, or silicone. 19.An electronic device comprising: a substrate having first and secondconductive traces on a first surface thereof, the first and secondconductive traces being separated on the substrate by a gaptherebetween; a light-emitting diode (LED) having first and seconddistinct electrical contacts on a first LED surface thereof, the firstand second contacts being attached and electrically coupled to,respectively, the first and second conductive traces; and a thermal pad,wherein (i) at least a portion of the thermal pad is disposed within thegap between the first and second conductive traces, (ii) at least aportion of the thermal pad is disposed between the first surface of thesubstrate and the LED, (iii) the thermal pad is electrically isolatedfrom at least one of the first or second electrical contacts, (iv) thethermal pad thermally couples the LED to the substrate withoutelectrically coupling the LED to the substrate, (v) the thermal pad hasa thermal conductivity greater than 1 W/(m·K), and (vi) the first andsecond contacts are bonded to the first and second conductive traces byan adhesive, the adhesive comprising at least one of apressure-activated adhesive, an anisotropic conductive adhesive, aconductive adhesive, a non-conductive adhesive, a heat-activatedadhesive, or a UV-activated adhesive.
 20. An electronic devicecomprising: a substrate having first and second conductive traces on afirst surface thereof, the first and second conductive traces beingseparated on the substrate by a gap therebetween; a light-emitting diode(LED) having first and second distinct electrical contacts on a firstLED surface thereof, the first and second contacts being attached andelectrically coupled to, respectively, the first and second conductivetraces; and a thermal pad, wherein (i) at least a portion of the thermalpad is disposed within the gap between the first and second conductivetraces, (ii) at least a portion of the thermal pad is disposed betweenthe first surface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) the first and second contacts are bonded to the first andsecond conductive traces by solder.
 21. An electronic device comprising:a substrate having first and second conductive traces on a first surfacethereof, the first and second conductive traces being separated on thesubstrate by a gap therebetween; a light-emitting diode (LED) havingfirst and second distinct electrical contacts on a first LED surfacethereof, the first and second contacts being attached and electricallycoupled to, respectively, the first and second conductive traces; and athermal pad, wherein (i) at least a portion of the thermal pad isdisposed within the gap between the first and second conductive traces,(ii) at least a portion of the thermal pad is disposed between the firstsurface of the substrate and the LED, (iii) the thermal pad iselectrically isolated from at least one of the first or secondelectrical contacts, (iv) the thermal pad thermally couples the LED tothe substrate without electrically coupling the LED to the substrate,(v) the thermal pad has a thermal conductivity greater than 1 W/(m·K),and (vi) the thermal pad is bonded to the first and second conductivetraces by an adhesive.
 22. The electronic device of claim 1, wherein thefirst and second conductive traces comprise at least one of aluminum,copper, gold, silver, carbon, silver ink, or chromium.
 23. Theelectronic device of claim 1, wherein (i) the LED is part of one of aplurality of light-emitting strings disposed on the first surface of thesubstrate and interconnected in an electric circuit, each light-emittingstring comprising a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string.
 24. The electronic deviceof claim 23, further comprising a power source for energizing theplurality of light-emitting strings and circuitry for controlling thecurrent through the plurality of light-emitting strings.
 25. Theelectronic device of claim 2, wherein the thermal pad comprises amaterial having a thermal conductivity in the range of about 75 W/(m·K)to about 600 W/(m·K).
 26. The electronic device of claim 2, wherein thesubstrate is flexible.
 27. The electronic device of claim 2, wherein thesubstrate comprises at least one of polyethylene naphthalate,polyethylene terephthalate, epoxy, polycarbonate, polyethersulfone,polyester, polyimide, polyethylene, or silicone.
 28. The electronicdevice of claim 2, wherein (i) the LED is part of one of a plurality oflight-emitting strings disposed on the first surface of the substrateand interconnected in an electric circuit, each light-emitting stringcomprising a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string.
 29. The electronic deviceof claim 28, further comprising a power source for energizing theplurality of light-emitting strings and circuitry for controlling thecurrent through the plurality of light-emitting strings.
 30. Theelectronic device of claim 3, wherein the thermal pad comprises amaterial having a thermal conductivity in the range of about 75 W/(m·K)to about 600 W/(m·K).
 31. The electronic device of claim 3, wherein thesubstrate is flexible.
 32. The electronic device of claim 3, wherein thesubstrate comprises at least one of polyethylene naphthalate,polyethylene terephthalate, epoxy, polycarbonate, polyethersulfone,polyester, polyimide, polyethylene, or silicone.
 33. The electronicdevice of claim 3, wherein (i) the LED is part of one of a plurality oflight-emitting strings disposed on the first surface of the substrateand interconnected in an electric circuit, each light-emitting stringcomprising a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string.
 34. The electronic deviceof claim 33, further comprising a power source for energizing theplurality of light-emitting strings and circuitry for controlling thecurrent through the plurality of light-emitting strings.
 35. Theelectronic device of claim 4, wherein the thermal pad comprises amaterial having a thermal conductivity in the range of about 75 W/(m·K)to about 600 W/(m·K).
 36. The electronic device of claim 4, wherein thesubstrate is flexible.
 37. The electronic device of claim 4, wherein thesubstrate comprises at least one of polyethylene naphthalate,polyethylene terephthalate, epoxy, polycarbonate, polyethersulfone,polyester, polyimide, polyethylene, or silicone.
 38. The electronicdevice of claim 4, wherein (i) the LED is part of one of a plurality oflight-emitting strings disposed on the first surface of the substrateand interconnected in an electric circuit, each light-emitting stringcomprising a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string.
 39. The electronic deviceof claim 38, further comprising a power source for energizing theplurality of light-emitting strings and circuitry for controlling thecurrent through the plurality of light-emitting strings.
 40. Theelectronic device of claim 12, wherein the thermal pad comprises amaterial having a thermal conductivity in the range of about 75 W/(m·K)to about 600 W/(m·K).
 41. The electronic device of claim 12, wherein thesubstrate is flexible.
 42. The electronic device of claim 12, whereinthe substrate comprises at least one of polyethylene naphthalate,polyethylene terephthalate, epoxy, polycarbonate, polyethersulfone,polyester, polyimide, polyethylene, or silicone.
 43. The electronicdevice of claim 12, wherein (i) the LED is part of one of a plurality oflight-emitting strings disposed on the first surface of the substrateand interconnected in an electric circuit, each light-emitting stringcomprising a plurality of LEDs spaced along and electricallyinterconnected in the light-emitting string.
 44. The electronic deviceof claim 43, further comprising a power source for energizing theplurality of light-emitting strings and circuitry for controlling thecurrent through the plurality of light-emitting strings.
 45. Theelectronic device of claim 1, further comprising an insulating layerdisposed between the thermal pad and the LED.
 46. The electronic deviceof claim 1, wherein the LED comprises a bare-die LED.
 47. The electronicdevice of claim 1, wherein the LED comprises a packaged LED.
 48. Theelectronic device of claim 1, wherein a surface area of the firstcontact is different from a surface area of the second contact.